1. Field of Invention
The present invention relates to a light emitting device control circuit and a short detection circuit thereof, in particular, to a light emitting device control circuit capable of detecting short circuit condition of light emitting devices and a short detection circuit for use therein.
2. Description of Related Art
FIG. 1 shows a schematic diagram of a typical power control circuit for driving LEDs (light emitting diodes). As shown in FIG. 1, the LED power control circuit 10 controls an LED array 20. The LED power control circuit 10 includes plural pins T11, T12, . . . , T1n, which are respectively connected to the negative terminals B of the LED strings to receive current or voltage signals therefrom, for feedback controlling the LED array 20. The positive terminal A of each LED string is connected to the supply voltage Vout.
In general, in the LED power control circuit 10, whether a short circuit condition occurs in an LED string can be detected according to whether the voltage at the negative terminal B is higher than a preset reference value. When the voltage at the negative terminal B of the LED string is higher than the preset reference value, a short circuit condition is detected. However, if the LED power control circuit 10 provides a dimming function, misjudgment may occur in such short circuit detection. Referring to FIG. 2, a dimming circuit 11 generates a PWM dimming signal, which has a PWM duty that turns ON and OFF the LED strings in each period to adjust the brightness of the LED strings. When the PWM dimming signal does not turn ON an LED string, no current flows through the LED string, and therefore there is no voltage drop across the LED string; thus, the voltage at the negative terminal B the LED string is close to the supply voltage Vout and higher than the preset reference value, which will cause a misjudgment in short circuit detection. To accurately determine whether the LED string is in a short circuit condition, 100% PWM duty is needed. That is, for an LED power control chip 10 with dimming function, whether any LED string in the LED array 20 is short-circuited can only be detected when the LEDs are in the maximum brightness. Therefore, the prior art LED power control chip 10 only has very limited capability in detecting the short circuit condition; in most cases, it is unable to detect the short circuit condition of the LED array 20.
In view of the above drawback of the prior art, the present invention proposes a light emitting device control circuit and a short detection circuit for use therein, which are capable of detecting short circuit condition of light emitting devices under various dimming conditions.